1. Technical Field
The present invention relates to an electrostatic protective circuit and a semiconductor device.
2. Related Art
FIG. 16 is a circuit schematic, illustrating a conventional electrostatic protective circuit. An electrostatic protective circuit 100 comprises bipolar transistors Q1 and Q2, serving as protective elements. In the bipolar transistor Q1 and Q2, collectors are mutually coupled. Further, an emitter and a base of the bipolar transistor Q1 are coupled to a signal line 102, and an emitter and a base of the bipolar transistor Q2 are coupled to a power supply (indicated as “GND” in FIG. 16). One end of the signal line 102 is coupled to an external terminal 104, and another end is coupled to an internal circuit (not shown).
When a positive signal potential is provided to the signal line 102 in such electrostatic protective circuit 100, a forward bias voltage is applied to a diode (first diode), which is composed of a collector-base junction of the bipolar transistor Q1, and a reverse bias voltage is applied to a diode (second diode), which is composed of a collector-base junction of the bipolar transistor Q2. Therefore, a positive potential of up to a breakdown voltage for the second diode can be applied to the signal line 102. On the other hand, when a negative signal potential is applied to the signal line 102, the first diode is inversely biased, and the second diode is forward biased. Therefore, a negative potential of up to a breakdown voltage for the first diode can be applied to the signal line 102.
In addition to above, prior art documents related to the present invention include: Japanese Patent Laid-Open No. 2002-50,640 and Japanese Patent Laid-Open No. 2006-100,532.
However, the present inventor has recognized that a dielectric breakdown of the protective element may be caused, even if the level of the signal potential is less than a breakdown voltage for a protective element (i.e., bipolar transistor Q1, Q2), when a signal potential input to the external terminal 104 is precipitously changed in the electrostatic protective circuit 100 of FIG. 16.
The details concerning such phenomenon will be described in reference to FIG. 17A and FIG. 17B. Abscissa of these graphs represent time t, and potential φ is shown in ordinate. In addition, a dotted line L1 and a dotted line L2 in the graphs illustrate the above-described breakdown voltages of the first and the second diodes, respectively. In FIG. 17A, waveforms of a signal potential and a potential at a node N are presented by a solid line and a dotted line, respectively. In FIG. 17B, a waveform of a voltage applied to the bipolar transistor Q1 (=signal potential−potential at node N) is presented.
First of all, the first diode is forward biased as the signal potential is increased (I), such that a potential at the node N (see FIG. 16) is also increased as following the increased signal potential. However, in this case, the potential at the node N in lower than the signal potential by only a certain level (around 0.5 V).
Next, when the signal potential is to be dropped (II), the potential at the node N is also to be dropped. However, as shown in FIG. 16, a parasitic capacitance C1 is created between the node N and the ground (indicated as “GND” in FIG. 16). Thus, when the signal potential is rapidly changed, the potential at the node N can not follow the change of the signal potential, due to the parasitic capacitance C1. Then, this results in the first diode being applied with a larger inverse bias voltage, and eventually the inverse bias voltage is increased to reach the breakdown voltage (BVcbo) for the first diode (III). As described above, even if the level of the signal potential is lower than the breakdown voltage for the diode, the level of the reverse bias applied to the diode can reach the above-described breakdown voltage, and this can lead to a dielectric breakdown.